The 6.5"x6.5" window is the largest and the most popular window for the Slimline 1 base unit. It's a great window to do sports bags, tote bags, brief cases and diaper bags. This window comes with two top pieces a silver and gold. The silver is for thinner materials and the gold is for thicker such as leather. You only use one top piece at a time. This window does not interchange with other clamp base units.